The present invention relates to a plate thickness measuring apparatus for a bottom plate of a cylindrical tank that measures plate thickness reduction of a bottom plate of a cylindrical tank such as a fuel tank.
Up to now, in Japan, a plate thickness measurement of a bottom plate of a cylindrical tank by ultrasonic waves has been performed according to the Notification of the Fire and Disaster Management Agency. That is, as for an annular plate within an inside range of 500 mm from an inside face of a side plate, it is performed at intervals of about 100 mm, for example, at fixed points designated in a staggered manner, and as for other portions of the annular plate and a bottom plate therein, it is performed at fixed points specified at intervals of about 1 m. Then, as the result of the plate thickness measurement of the bottom plate at the above fixed points, if a problem arises where a decrement of plate thickness is detected beyond a reference value, plate thickness measurement is further performed for specified positions at intervals of 30 mm, in a range of 300 mm radius with the position as a center by using ultrasonic waves, and hence the plate thickness reduction of the tank bottom near the problem position is known.
However, the following problems that should be solved exist in the conventional plate thickness measurement of the bottom plate of a cylindrical tank.
(1) In the plate thickness measurement of the tank bottom plate using ultrasonic waves, since acoustic velocities in a coating film portion and a steel plate portion differ greatly, if the measurement is performed from an upper surface of the coating film by using a general ultrasonic thickness gauge, a very large measurement error arises in the determination of the thickness of an actual bottom plate, i.e., steel plate thickness. Although a multi-echo type ultrasonic thickness gauge is developed as a counter-measure for this, if there is backside corrosion, it is difficult to correctly determine the state of the steel plate thickness reduction by the corrosion of the tank bottom plate since a multi-echo may not be obtained.
(2) Therefore, on the occasion of measurement of steel plate thickness, a method is adopted in which plate thickness measurement is performed for only the steel plate portion after removing a coating film portion and a recoat is performed after the measurement, however, there is a problem in that an idle period of a tank becomes long due to the measurement and an economical problem that costs of coating film removal and recoat arise in addition to measurement cost.
(3) Furthermore, there is a problem that, if an unusual plate thickness reduction of the steel plate is not detected by the measurement at spaced fixed points, such as unusual plate thickness reduction is ignored even if it actually exists in a portion except the fixed points. It is thus impossible to detect the plate thickness reduction of the steel plate by local corrosion, and hence it is not possible to grasp the plate thickness reduction of the steel plate covering the whole tank bottom plate.
(4) In addition, there is a problem that plate thickness measurement of the tank bottom plate cannot be performed simply and quickly since the measurement is performed at new fixed points, which are given by performing subdivision with a fixed point as a center, if an unusual plate thickness reduction is detected in the fixed point.
The present invention aims to provide a plate thickness measuring apparatus for the bottom plate of a cylindrical tank which can determine plate thickness reduction of a steel plate covering the whole tank bottom plate without removing the coating of the tank bottom plate.
A plate thickness measuring apparatus for a bottom plate of a cylindrical tank according to the present invention for attaining the above object has: a measuring cart which travels on a tank bottom plate to be measured; auxiliary carts which are arranged in the width direction of the measuring cart and are attached to the measuring cart through universal joint mechanisms; reflection type ultrasonic probes which measure the thickness of the tank bottom plate, and coating film thickness gauges which measure the thickness of a coating film on a surface of the tank bottom plate, both of which are attached in each of the auxiliary carts; a rotary encoder which is attached to the measuring cart and measures the traveling distance of the measuring cart; and an operation and display unit that inputs the outputs of the ultrasonic probes, the outputs of the coating film thickness gauges, and an output of the rotary encoder, measures and stores the actual thickness of the tank bottom plate that is obtained by subtracting the thickness of the coating film at a specific position of the tank bottom plate based on the outputs of the ultrasonic probes and the outputs of the coating film thickness gauges. Furthermore, the apparatus specifies the position by the rotary encoder, displays the relation between the position of the bottom plate and the thickness thereof on a screen and stores it.
In the plate thickness measuring apparatus for the bottom plate of a cylindrical tank according to the present invention, it is desirable that plural reflection type ultrasonic probes are arranged in the width direction of the measuring cart so that the plate thickness measurement can be performed in the whole width direction of the measuring cart, whereby it becomes possible to measure the plate thickness of almost all the portions of the tank bottom plate by making the measuring cart travel on the tank bottom plate. In this case, it is preferable that the auxiliary carts where the ultrasonic probes are mounted are arranged in plural rows in the traveling direction of the measuring cart, and that the auxiliary carts in each row are arranged in a staggered or step-like pattern in a plan view.
In addition, in the plate thickness measuring apparatus for the bottom plate of a cylindrical tank according to the present invention, it is preferable that the reflection type ultrasonic probe is comprised of a double crystal probe having a transmitting element and a receiving element, and that the coating film thickness gauge is comprised of an eddy current type sensor. It is thus possible to measure and evaluate a state of the plate thickness reduction by the local corrosion of the steel plate over the whole tank bottom plate with high precision without removing the coating of the tank bottom plate.
In the plate thickness measuring apparatus for the bottom plate of a cylindrical tank according to the present invention, it is preferable that an operation handle provided in the measuring cart is detachable, and hence, it is possible to measure the thickness of the tank bottom plate with the measuring cart passing under piping provided above the tank bottom plate. Therefore, even if piping which would be obstructive to the measurement is in a tank, it is possible to measure the thickness of the bottom plate by passing thereunder.